Propene (C3H6), often also referred to as propylene, is one of the most important starting substances of the chemical industry. The demand for the base material propylene is increasing worldwide, wherein propylene just like ethylene mostly is produced from petroleum in a steam cracker in a ratio dependent on the process conditions and the raw materials.
To obtain additional propylene, a number of processes exist, such as the propane dehydrogenation which proceeds from propane as educt. What is known, however, above all is the so-called MTP (Methanol-to-Propylene) process, in which so-called oxygenates such as methanol (MeOH) or dimethyl ether (DME) are converted to olefins by catalytic conversion on a zeolitic catalyst. By varying the catalyst and the process conditions, the selectivity of the products obtained can be influenced and the product spectrum thus can be shifted towards short-chain olefins (then often also the process name Methanol-to-Olefin (MTO)), towards longer-chain products (then often also the process name Methanol-to-Gasoline (MTG)) or towards propylene.
The fundamentals of an MTP process are described for example in the document DE 10 2005 048 931 A1. From an educt mixture containing steam and oxygenates, such as methanol and/or dimethyl ether, C2 to C4 olefins are produced above all. By a heterogeneously catalyzed reaction in at least one reactor, the educt mixture is converted to a reaction mixture comprising low-molecular olefins and gasoline hydrocarbons. By a suitable separation concept, higher olefins, above all the C5+ fraction, can at least partly be recirculated into the reactor as recycling stream and in said reactor for the most part be converted to propylene, whereby the yield of propylene is increased.
The MTP process usually has a propylene yield of about 65% (mole C). An increased propylene yield would distinctly improve the economy of the process. As predominant by-product in the MTP process a gasoline-like mixture (MTP gasoline) is obtained, which substantially consists of olefins, paraffins, cyclic hydrocarbons and aromatics. This MTP gasoline likewise can be incorporated into a succeeding value chain, but has a lower market price than propylene.
As described in the document WO 2006/136433 A1 it therefore is attempted for example to subject the MTP gasoline or fractions of the same to a post-processing in the form of an olefin metathesis, in which the MTP gasoline or fractions obtained therefrom is converted on a zeolitic catalyst at temperatures of about 400 to 500° C. and a pressure of 1 to 5 bar. Due to this downstream reaction, a moderate increase of the propylene yield of the entire process can be achieved, but the total yield still lies below 70 mol-%.
A direct recirculation of the MTP gasoline into the MTP reactor provides no increase in the yield of propylene. Since undesired alkylation reactions of the aromatics occur inside the MTP reactor, which consume methanol which then no longer is available for the selective formation of propylene, the propylene yield of the entire process even would decrease in the case of the direct recirculation of the MTP gasoline into the MTP reactor.
Some processes therefore aim at converting the heavier olefins obtained, so that at least a product with homogeneous composition and higher market price is obtained. The patent specification U.S. Pat. No. 4,543,435 for example teaches that at least a part of the olefins obtained is to be converted to heavy hydrocarbons, so that the yield of liquefied gas and gasoline can be increased within the MTP process.
WO 2011/131647 describes a process for producing aromatic hydrocarbons, in which a feed of light alkanes is at least partly converted to aromatics on a suitable catalyst. Parallel thereto an MTO process takes place. A part of the oxygenate feed of the MTO process is produced in that the hydrogen obtained during the conversion of the alkanes to aromatics is converted to an oxygenate with carbon monoxide and/or carbon dioxide. The product streams thus obtained can easily be combined with the other by-products of the MTP process, above all methane, carbon oxides, hydrogen and a product similar to liquefied gas.
For increasing the yield of valuable products from an MTP process, a hydrogenation of the aromatic hydrocarbons obtained also is known. US 2004/0039239 for example teaches that higher olefins are to be hydrogenated on a suitable hydrogenation catalyst. In particular due to the fact that aromatics also are hydrogenated to paraffins, the yield of a gasoline-like valuable product thus can be increased.
From the patent specification U.S. Pat. No. 4,482,772 a hydrogenation within an MTP process is known, in which first the conversion of the oxygenates to olefins takes place and the olefins thus obtained subsequently are oligomerized. Subsequent to the oligomerization, at least parts of the product stream are hydrogenated, whereby aromatics contained in the product stream are converted to cycloparaffins. The yield of a gasoline-like valuable product likewise can be increased thereby.
For carrying out such hydrogenations, various types of catalyst and their possible applications are known for example from the patent application US 2007/0284284 A1.
Finally, the International Patent Application WO 2014/124844 A1 proposes a modified MTP process in which an aromatics fraction is obtained and the same is converted into a fraction containing cycloparaffins by hydrogenation. The latter is recirculated to the olefin synthesis reactor as recycle stream and in said reactor partly converted to olefins, among them propylene. In this way an increase of the propylene yield is achieved already, but reactive components, in particular C5+ olefins, still remain unused for the formation of propylene and for example are admixed to the MTP gasoline. Another part of the C5+ olefins is withdrawn from the process via purge streams, whose actual task it is to prevent an enrichment of the C5+ paraffins in the MTP process.